1. Field of the Invention
This invention relates to a method and apparatus for coding image data and, more particularly, to binary coding of image data and compression of image data in image transmission and image filing.
2. Description of the Prior Art
Image data compression in the prior art involves a variety of techniques for the purpose of shortening data transmission time and reducing data file capacity. In the field of facsimile transmission, the modified Huffman method (MH method) of an encoding system using the run length of black-and-white pixels, which is one characterizing quantity of an image, is employed as a compression technique. However, occasions on which half-tone images such as photographs are transmitted or filed are increasing.
The binary coding of a half-tone image by the dither method generally employed in the prior art is known to be unsuitable for compression techniques such as MH coding since the run of a binary-coded image is divided. For this reason, various improvements have been attempted focusing on the periodicity of dither image data.
An error diffusion method is available as another method of reproducing half tones. This method, which entails dispersing a concentration error that occurs in binary coding into unprocessed peripheral pixels and then proceeding with binary coding, is the focus of much attention recently since it provides better tonality and resolution than does the dither method. However, the image data processed by the error diffusion method do not possess any conspicuous periodicity in addition to the fact that the run is divided. When these image data are compressed as by MH coding, the amount of data increases by a factor of about 1.5 to 2 instead.
Another method that has been considered, as shown in FIG. 10, involves binary coding a digital signal from input means 80 by digitizing means 81, and coding missed pixels, which are predicted by predicting means 82 , by a coder 83. However, even in this case the run of missed pixels is short, as a result of which data compression cannot be performed by MH coding.